(a) Technical Field
The present invention provides a method of preparing lactide with a high yield from lactate, which are obtained through fermentation.
(b) Background Art
Since the 20th century, rapid global industrialization has caused the global economy largely dependent upon fossil fuel resources, particularly, petroleum resources. It has been noted that consumption of petroleum resources has been considerably increased along with the rapid development of industries and global population growth. However, petroleum is not a recyclable resource, and its reserves are limited. In addition, the carbon dioxide emission from the consumption of fossil fuels has been blamed as a main cause for global warming. It has been the focus of researches in the related industries to improve the efficiency of the petroleum economy, and to reduce the carbon dioxide emission.
Recently, plant-derived (i.e., biomass) polymers have received public attention due to their eco-friendliness, particularly, with reduced carbon dioxide emissions (rather than biodegradability). The biomass polymers are produced from recyclable plant resources (such as, corns, beans, sugar canes, and woods) by using chemical or biological methods. Among the biomass polymers, polylactic acid, a linear aliphatic polyester, is a carbon neutral and eco-friendly thermoplastic polymer. Polylactic acid is produced by polymerizing monosaccharides obtained by starch-fermenting corns and potatoes, or produced by saccharifying and fermenting plant-derived cellulose.
Despite various advantages that polylactic acid offers, polylactic acid however has not been considered to be suitable to be used in vehicles, due to its expensive manufacturing costs compared with petrochemical-derived polymers. In addition, industrial applications of polylactic acid resins are limited due to their poor physical properties. To be used in automotive chassis and engines, polylactic acid resins need to be modified to have improved physical properties, such as, a high thermal resistance and an impact strength.
To achieve these goals, there is a known technique of preparing stereo-complex type resins in the art. The technique involves blending enantiomer resins. To develop this technique, L-polylactic acid and D-polylactic acid need to be manufactured in an economical manner. Specifically, in the preparation of polylactic acid, resins are manufactured a process comprising steps of converting fermented and purified lactic acid into lactide, and a ring-opening polymerization of the lactide. In this process, there is a need to economically improve the step of converting lactic acid to lactide.
Further, during the fermentation step, the lactic acid that is used to prepare polylactic acid exists in a salt form, for example, ammonium lactate (NH4+Lac−) and sodium lactate (Na+ Lac−). This occurs because a base is added during the fermentation step to adjust acidity. Accordingly, a process of manufacturing lactic acid comprises concentrating lactate in the form of a salt and converting the concentrated lactate in the form of a salt to obtain pure lactic acid. Clearly, the manufacturing costs are increased due to cost associated with the preparation of pure lactic acid. Therefore, if lactide can be produced from lactate instead of from pure lactic acid, the manufacturing cost of lactide would be reduced. However, up to today, such a process with an optimized technique has not yet been developed to achieve a satisfactory level.
In the art, solid-phase lactide is generally prepared from liquid-phase lactic acid that is obtained through fermentation, purification, and concentration processes. Accordingly, there is a need to develop a process or catalyst thereof that may be used to economically prepare lactide from liquid-phase lactic acid.
So far, the majority of the methods for manufacturing lactide comprise polymerizing low molecular weight polylactic acid obtained through fermentation, purification, and concentration, and depolymerizing the low molecular weight polylactic acid at a high temperature, so that cyclo lactide is prepared by back biting in low molecular weight polylactic acid chains.
For example, Korean Patent Application Publication No. 2008-18657 discloses a method of preparing lactide for synthesizing L-lactic acid from L-, D-, or DL-lactic acid, the method comprising the steps of: (1) synthesizing low molecular weight lactic acid from lactic acid; and (2) preparing crude lactide by heating and pressing the low molecular weight lactic acid synthesized in the step (1) in the presence of a catalyst, wherein the catalyst is a metal oxide catalyst including a metal selected from the group consisting of Sn, Zn, Fe, and Sb.
Korean Patent Application Publication No. 2008-18650 discloses a method of isolating chirally pure D- and L-lactides. The method includes: synthesizing low molecular weight lactic acid from lactic acid; preparing crude lactide from the low molecular weight lactic acid in the presence of a catalyst; adding the crude lactide to a solvent to obtain a solution of the crude lactide; stirring the solution at a constant temperature; centrifuging the stirred solution; and drying the centrifuged sample.
Korean Patent No. 171431 discloses a method of preparing lactide from an aqueous lactic acid feed, particularly, with an enriched L1A, L2A, L3A, or a mixture thereof. The method comprise the steps of: (a) converting an aqueous lactic acid feed to its vapor phase by evaporating the lactic acid feed in a hot gas stream; (b) passing the feed vapors through a vapor phase reaction zone maintained at elevated temperature; and (c) withdrawing from the reaction zone lactide, water, and unreacted aqueous lactic acid feed.
Further, Japanese Patent Application Publication No. 1995-165753 discloses a method of purifying lactide by removing meso-lactide from a mixture of L-lactide and/or D-lactide, and mesolactide. The method includes a step of bringing the mixture into contact with water. Moreover, Japanese Patent Application Publication No. 2004-149418 discloses a method of purifying lactide through steps including bringing a solidified crude lactide into contact with water, separating the solidified crude lactide into an aqueous phase and a solid phase, and recovering the solid phase.
In general, these methods are performed by preparing low molecular weight polylactic acid, and then performing pyrolysis to prepare lactide. Since a heat treatment at a high temperature (i.e., greater than 230° C.) is performed and gaseous lactide is collected by using low temperature trap, the processes are complex and uneconomical.
U.S. Pat. No. 5,750,732 discloses a method of producing a cyclo ester by mixing lactic acid, an organic solvent (such as, xylene), and a catalyst (such as, sulfonic acid) to obtain a mixture, and heating and refluxing the mixture to prepare lactide. Japanese Patent Application Publication No. 1994-0031175 discloses a catalyst useful for preparing lactide having low hygroscopicity in a high yield. The preparation involves using lactic acid prepolymer or lactic ester and a catalyst without a high temperature heat treatment. European Patent Nos. 261,572 and 275,581 disclose another type of catalysts, such as, tin powder, tin halide, and tin carboxylate. Moreover, UK Patent No. 1,007,347 discloses the use of tin alkoxide as a catalyst. Korean Patent No. 2010-5820 discloses a method of synthesizing chirally pure lactide. The method includes: (A) dissolving lactic acid or alkyl lactate in an organic solvent; (B) adding an enzyme to the organic solution of the lactic acid or alkyl lactate and stirring for 4 to 24 hours; and (C) separating chirally pure lactide.
However, these existing techniques in the field are not economical s as they are generally complicated or have low yields.